Hi ​@sunnywaygis 
Can you verify that your ArcGIS Enterprise/Portal is configured to use SAML authentication? 
You can verify this information from your GIS Administrator

or you can simply check in your ArcGIS Enterprise by browsing to this location 
Organization → Settings → Security → Logins 

Since the error is 401 Unauthorized .. It is prudent to ask if the USER trying to access the Portal have permission to connect using OAuth or not. 

From Esri documentation: 
Exception representing a 401 - Unauthorized response HTTP error. This HTTP error occurs when a network request is made to a NTLM-secured service, with no or invalid username/password.

Can you review some Log files from ArcGIS Enterprise to check if there is any useful information? 

Thanks Rahul for your reply. We use IWA for authentication within our Portal. I also checked Portal logs, don't see specific error for user in portal logs. I am able to login to Portal site without any issues. Does it have to SAML authentication for FME Form to connect to Portal? 

Appreciate any feedback. 

Thanks 

Hi ​@sunnywaygis 
To use Web Services, OAuth currently only support ArcGIS Build-in or SAML authentication. 

If you have your portal configured with IWA (web authentication), you can access the service by setting the authentication to either Kerberos or NTLM 

Thanks Rahul, I was able to connect to services using NTLM method on the reader. I really appreciate your help.